Enhanced ER-diagram

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Enhanced ER-diagram
CS157A Lecture 5

• Prof. Sin-Min Lee
• Department of Computer Science

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• Example A library database contains a listing
  of authors that have written books on various
  subjects (one author per book). It also contains
  information about libraries that carry books on
  various subjects.

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RELATIONSHIPS (Cont)

• Example A library database contains a listing
  of authors that have written books on various
  subjects (one author per book). It also contains
  information about libraries that carry books on
  various subjects.
• Entity sets authors, subjects, books, libraries

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RELATIONSHIPS (Cont)

• Example A library database contains a listing
  of authors that have written books on various
  subjects (one author per book). It also contains
  information about libraries that carry books on
  various subjects.
• Entity sets authors, subjects, books, libraries
• Relationship sets wrote, carry, indexed

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RELATIONSHIPS (Cont)
Subject matter
title
isbn
SS
index
wrote
books
subject
authors
name
carry
libraries
address
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RELATIONSHIPS (Cont)
Subject matter
title
isbn
SS
index
wrote
books
subject
authors
name
carry
quantity
libraries
address
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BINARY RELATIONSHIP

• A binary relationship between entity set A and B
  might be
• 11 Women marrying Men (function)
• N1 Children having mothers (function)
• 1N Mothers having children (inverse function)
• MN Students enrolled in a class

marry
B
A
women
men
n
1
B
A
having
children
mothers
n
1
having
B
A
mothers
children
enrolled
B
A
students
classes
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KEY

• Entities and relationships are distinguishable
  using various keys
• A key is a combination of one or more attributes,
  e.g., social-security number, combination of name
  and social-security number.
• A superkey is a key defined either for an entity
  set or relationship set that uniquely identifies
  an entity, e.g., social-security number, phone
  number, combination of name and social-security
  number.
• A candidate key is a minimal superkey that
  uniquely identifies either an entity or a
  relationship, e.g., social-security number, phone
  number.
• A primary key is a candidate key that is chosen
  by the database designer to identify the entities
  of an entity set.

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• A foreign key is a set of one or more attributes
  of a strong entity set that are employed to
  construct the discriminator of a weak entity set.
  The primary key of a weak entity set is formed
  by the primary key of the strong entity set on
  which it is existence-dependent.
• Relationship sets also have primary keys. Assume
  R is a relationship set involving entity sets E1,
  E2, ..., En. Let primary-key(Ei) denote the
  primary key for entity set Ei. Assume
  primary-key(Ei) is unique for 1 i n. If R has
  no attributes then its superkey is
• primary-key(E1) ? primary-key(E2) ? ... ?
  primary-key(En)
• This is a primary key if the mapping constraint
  is many-to-many.
• If the mapping constraint is many to one from E1
  to E2 then the primary key of R is primary key of
  E1.

title
SS
wrote
books
authors
name
edition
R
E1
E2
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EXAMPLE

• Employees of a large company, e.g., IBM, where an
  employee reports to a manager. The manager is
  also an employee who reports to another manager.
  This chain of command continues to the very top
  where the CEO is the only employee who is not
  reporting to a manager. Draw the ER diagram for
  this example.

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Works for
SS
Emp
name
address
Primary keys Emp SS Works-for (empSS,
mgrSS)
Works for
SS
Emp
name
Primary keys Emp SS Works-for (empSS)
address
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• A relationship may involve n entities, N-ary
  relationship
• It is always possible to replace a non-binary
  relationship set by a number of distinct binary
  relationship sets

fans
Football Teams
People
on
Date
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• Example A library database contains a listing
  of authors that have written books on various
  subjects (one author per book). It also contains
  information about libraries that carry books on
  various subjects.
• Entity sets authors, subjects, books, libraries
• Relationship sets wrote, carry, indexed
• E-R diagram

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Enhanced-ER (EER) Model Concepts

• Includes all modeling concepts of basic ER
• Additional concepts subclasses/superclasses,
  specialization/generalization, categories,
  attribute inheritance
• The resulting model is called the enhanced-ER or
  Extended ER (E2R or EER) model
• It is used to model applications more completely
  and accurately if needed
• It includes some object-oriented concepts, such
  as inheritance

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Subclasses and Superclasses (1)

• An entity type may have additional meaningful
  subgroupings of its entities
• Example EMPLOYEE may be further grouped into
  SECRETARY, ENGINEER, MANAGER, TECHNICIAN,
  SALARIED_EMPLOYEE, HOURLY_EMPLOYEE,
• Each of these groupings is a subset of EMPLOYEE
  entities
• Each is called a subclass of EMPLOYEE
• EMPLOYEE is the superclass for each of these
  subclasses
• These are called superclass/subclass
  relationships.
• Example EMPLOYEE/SECRETARY, EMPLOYEE/TECHNICIAN

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Subclasses and Superclasses (2)

• These are also called IS-A relationships
  (SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A
  EMPLOYEE, ).
• Note An entity that is member of a subclass
  represents the same real-world entity as some
  member of the superclass
• The Subclass member is the same entity in a
  distinct specific role
• An entity cannot exist in the database merely by
  being a member of a subclass it must also be a
  member of the superclass
• A member of the superclass can be optionally
  included as a member of any number of its
  subclasses
• Example A salaried employee who is also an
  engineer belongs to the two subclasses ENGINEER
  and SALARIED_EMPLOYEE
• It is not necessary that every entity in a
  superclass be a member of some subclass

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Attribute Inheritance in Superclass / Subclass
Relationships

• An entity that is member of a subclass inherits
  all attributes of the entity as a member of the
  superclass
• It also inherits all relationships

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Specialization

• Is the process of defining a set of subclasses of
  a superclass
• The set of subclasses is based upon some
  distinguishing characteristics of the entities in
  the superclass
• Example SECRETARY, ENGINEER, TECHNICIAN is a
  specialization of EMPLOYEE based upon job type.
• May have several specializations of the same
  superclass
• Example Another specialization of EMPLOYEE based
  in method of pay is SALARIED_EMPLOYEE,
  HOURLY_EMPLOYEE.
• Superclass/subclass relationships and
  specialization can be diagrammatically
  represented in EER diagrams
• Attributes of a subclass are called specific
  attributes. For example, TypingSpeed of SECRETARY
• The subclass can participate in specific
  relationship types. For example, BELONGS_TO of
  HOURLY_EMPLOYEE

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Example of a Specialization
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Generalization

• The reverse of the specialization process
• Several classes with common features are
  generalized into a superclass original classes
  become its subclasses
• Example CAR, TRUCK generalized into VEHICLE
  both CAR, TRUCK become subclasses of the
  superclass VEHICLE.
• We can view CAR, TRUCK as a specialization of
  VEHICLE
• Alternatively, we can view VEHICLE as a
  generalization of CAR and TRUCK

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Generalization and Specialization

• Arrow pointing to the generalized superclass
  represents a generalization
• Arrows pointing to the specialized subclasses
  represent a specialization
• We do not use this notation because it is often
  subjective as to which process is more
  appropriate for a particular situation
• We advocate not drawing any arrows in these
  situations
• A superclass or subclass represents a set of
  entities
• Shown in rectangles in EER diagrams (as are
  entity types)
• Sometimes, all entity sets are simply called
  classes, whether they are entity types,
  superclasses, or subclasses

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Constraints on Specialization and Generalization
(1)

• If we can determine exactly those entities that
  will become members of each subclass by a
  condition, the subclasses are called
  predicate-defined (or condition-defined)
  subclasses
• Condition is a constraint that determines
  subclass members
• Display a predicate-defined subclass by writing
  the predicate condition next to the line
  attaching the subclass to its superclass
• If all subclasses in a specialization have
  membership condition on same attribute of the
  superclass, specialization is called an attribute
  defined-specialization
• Attribute is called the defining attribute of the
  specialization
• Example JobType is the defining attribute of the
  specialization SECRETARY, TECHNICIAN, ENGINEER
  of EMPLOYEE
• If no condition determines membership, the
  subclass is called user-defined
• Membership in a subclass is determined by the
  database users by applying an operation to add an
  entity to the subclass
• Membership in the subclass is specified
  individually for each entity in the superclass by
  the user

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Constraints on Specialization and Generalization
(2)

• Two other conditions apply to a
  specialization/generalization
• Disjointness Constraint
• Specifies that the subclasses of the
  specialization must be disjointed (an entity can
  be a member of at most one of the subclasses of
  the specialization)
• Specified by d in EER diagram
• If not disjointed, overlap that is the same
  entity may be a member of more than one subclass
  of the specialization
• Specified by o in EER diagram
• Completeness Constraint
• Total specifies that every entity in the
  superclass must be a member of some subclass in
  the specialization/ generalization
• Shown in EER diagrams by a double line
• Partial allows an entity not to belong to any of
  the subclasses
• Shown in EER diagrams by a single line

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Constraints on Specialization and Generalization
(3)

• Hence, we have four types of specialization/genera
  lization
• Disjoint, total
• Disjoint, partial
• Overlapping, total
• Overlapping, partial
• Note Generalization usually is total because the
  superclass is derived from the subclasses.

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Example of disjoint partial Specialization
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Specialization / Generalization Hierarchies,
Lattices and Shared Subclasses

• A subclass may itself have further subclasses
  specified on it
• Forms a hierarchy or a lattice
• Hierarchy has a constraint that every subclass
  has only one superclass (called single
  inheritance)
• In a lattice, a subclass can be subclass of more
  than one superclass (called multiple inheritance)

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• In a lattice or hierarchy, a subclass inherits
  attributes not only of its direct superclass, but
  also of all its predecessor superclasses
• A subclass with more than one superclass is
  called a shared subclass
• Can have specialization hierarchies or lattices,
  or generalization hierarchies or lattices
• In specialization, start with an entity type and
  then define subclasses of the entity type by
  successive specialization (top down conceptual
  refinement process)
• In generalization, start with many entity types
  and generalize those that have common properties
  (bottom up conceptual synthesis process)
• In practice, the combination of two processes is
  employed

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Specialization / Generalization Lattice Example
(UNIVERSITY)
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Categories (UNION TYPES)

• All of the superclass/subclass relationships we
  have seen thus far have a single superclass
• A shared subclass is subclass in more than one
  distinct superclass/subclass relationships, where
  each relationships has a single superclass
  (multiple inheritance)
• In some cases, need to model a single
  superclass/subclass relationship with more than
  one superclass
• Superclasses represent different entity types
• Such a subclass is called a category or UNION
  TYPE

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• Example Database for vehicle registration,
  vehicle owner can be a person, a bank (holding a
  lien on a vehicle) or a company.
• Category (subclass) OWNER is a subset of the
  union of the three superclasses COMPANY, BANK,
  and PERSON
• A category member must exist in at least one of
  its superclasses
• Note The difference from shared subclass, which
  is subset of the intersection of its superclasses
  (shared subclass member must exist in all of its
  superclasses).

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Example of categories(UNION TYPES)
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Formal Definitions of EER Model (1)

• Class C A set of entities could be entity type,
  subclass, superclass, category.
• Subclass S A class whose entities must always be
  subset of the entities in another class, called
  the superclass C of the superclass/subclass (or
  IS-A) relationship S/C
• S ? C
• Specialization Z Z S1, S2,, Sn a set of
  subclasses with same superclass G hence, G/Si a
  superclass relationship for i 1, ., n.
• G is called a generalization of the subclasses
  S1, S2,, Sn
• Z is total if we always have
• S1 ? S2 ? ? Sn GOtherwise, Z is partial.
• Z is disjoint if we always have
• Si n S2 empty-set for i ? j
• Otherwise, Z is overlapping.

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Formal Definitions of EER Model (2)

• Subclass S of C is predicate defined if predicate
  p on attributes of C is used to specify
  membership in S that is, S Cp, where Cp is
  the set of entities in C that satisfy p
• A subclass not defined by a predicate is called
  user-defined
• Attribute-defined specialization if a predicate
  A ci (where A is an attribute of G and ci is a
  constant value from the domain of A) is used to
  specify membership in each subclass Si in Z

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• Note If ci ? cj for i ? j, and A is
  single-valued, then the attribute-defined
  specialization will be disjoint.
• Category or UNION type T
• A class that is a subset of the union of n
  defining superclasses D1, D2,Dn, ngt1
• T ? (D1 ? D2 ? ? Dn)A predicate pi on the
  attributes of T.
• If a predicate pi on the attributes of Di can
  specify entities of Di that are members of T.
• If a predicate is specified on every Di T
  (D1p1 ? D2p2 ?? Dnpn
• Note The definition of relationship type should
  have 'entity type' replaced with 'class'.

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Specialization

• This is the process of maximising the differences
  between members of an entity by identifying their
  distinguishing characteristics.
• Staff(staff_no,name,address,dob)
• Manager(bonus)
• Secretary(wp_skills)
• Sales_personnel(sales_area, car_allowance)

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GENERALIZATION AND SPECIALIZATION

• Generalization is the result of computing the
  union of two or more entity sets to produce a
  higher-level entity set. It represents the
  containment relationship that exists between the
  higher-level entity set and one or more
  lower-level entity sets.
• Specialization constructs the lower level entity
  sets that are a subset of a higher level entity
  set.

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GENERALIZATION AND SPECIALIZATION
sid
student
name
is A
Specialization
Generalization
graduate
Undergrad
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GENERALIZATION AND SPECIALIZATION

• Undergrad and graduate are termed subclasses of
  the superclass student.
• This is a superclass/subclass or simply
  class/subclass relationship.
• A member of a subclass MUST be a member of the
  superclass.
• An alternative notation is the Union symbol
• The circle with d specifies that the
  specializations are disjoint. A member of
  Undergrad entity set may NOT be a member of the
  graduate entity set.

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GENERALIZATION AND SPECIALIZATION
sid
student
name
d
Specialization
Generalization
U
U
graduate
Undergrad
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• A design may require all members of an entity-set
  to be specialized. For example, an employee MUST
  be a member of either a Salaried or Part-time.
  Use double lines to dictate this constraint

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GENERALIZATION AND SPECIALIZATION
sid
Emp
name
d
Specialization
Generalization
U
U
Part-time
Salaried
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GENERALIZATION AND SPECIALIZATION

• One may allow the specialized entity sets to
  overlap. For example, an entity might be both a
  Salaried and Part-time. o stands for Overlap
  when specializing.

sid
Emp
name
o
Specialization
Generalization
U
U
Part-time
Salaried
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Generalisation

• Generalisation is the process of minimising the
  differences between entities byidentifying common
  features.
• This is the identification of a generalised
  superclass from the original subclasses. This is
  the process of identifying the common attributes
  and relationships.

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UML Example for Displaying Specialization /
Generalization
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Alternative Diagrammatic Notations
Symbols for entity type / class, attribute and
relationship
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Notations for displaying specialization /
generalization
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Displaying attributes
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Various (min, max) notations
Displaying cardinality ratios